Two stage cascaded converters for converting a digital word into a corresponding analog voltage are known. The first stage includes a series string, or ladder of resistors coupled across a supply voltage for dividing the voltage into a number of incremental voltages. The first resistor ladder resolves higher order bits of the digital word and the second resistor ladder resolves lower order bits. The first resistor ladder is coupled to the second resistor ladder by a plurality of switches and buffer amplifiers. Such arrangements are shown in U.S. Pat. Nos. 4,543,560 and 3,997,892. Generally, buffer amplifiers have offset voltages associated therewith and require trimming in order to obtain satisfactory conversion accuracy.
Another type of digital to analog converter utilizes a Kelvin-Varley voltage divider in which the accuracy of the conversion is dependent on the accuracy of resistor matching. More particularly, the Kelvin-Varley divider includes a plurality of resistor ladders coupled together by a plurality of switches with each of the resistors having a resistance value precisely matched to the value of the other resistors. A portion of each resistor ladder is connected in parallel with a portion of a succeeding resistor ladder and the loading of preceding ladders is critical to obtaining a desired output voltage. Also, switch resistance is critical and must be extremely low.